Does anyone know of an institution with similar standing of IOSH but in Australia. I am due to move across there to work in July and want to join a group that can keep me abreast of local and new legislation etc.

IOSH have long-standing links with SIA. One of the challenges here (I'm currently visiting relatives, though have never worked in Oz) is the very large distances that often make regular face-to-face meetings difficult.

Obviously OSH legislation is important (both federal and state - so very different from UK) but I suggest the main purpose of a professional body is much more than that. The state Worksafe websites are a good starting point if you're just interested in the legislation. Don't know which sector you will be joining, but both mining and offshore in Oz have strong 'best practice' cultures that mean many organisations are more interested in OSH excellence than compliance.